1. Field of the Invention
Embodiments of the invention relate to ballast circuits for powering gas discharge lamps such as fluorescent lamps.
2. Related Technology
Gas discharge lamps such as fluorescent lamps are driven by alternating current (AC) drive signals. The circuits used to produce the drive signals are typically referred to as ballasts. Conventional ballasts are typically analog resonators that convert a DC voltage to an AC voltage having an appropriate frequency for driving a lamp.
FIG. 1 shows a first example of a conventional analog ballast. In this circuit, an AC line voltage is converted to DC in a rectifier 10, which provides rail and ground voltages for the ballast circuit. A drive signal for driving a gas discharge lamp 12 is generated by an analog drive circuit 14 that controls the voltage at a control node 16 to which the gates of complementary MOSFETs 18, 20 are coupled. The frequency produced by the analog drive circuit 14 is a function of resonant elements within the drive circuit, resonant elements in the discharge lamp 12, an inductor coupled between the common node 24 of the switches 18, 20, and a control inductor 26 provided between the drive circuit 14 and the control node 16. To provide dimming, the control inductor 26 is mutually coupled to an inductor provided within a control circuit 28. The control circuit 28 receives a lamp current feedback signal from a current sensor 30 that is proportional to the lamp current, and receives a control signal from a user control device such as a potentiometer. Based on a comparison of the lamp current signal and the control signal, the control circuit 28 controls the voltage across its inductor, which changes the load across the control inductor 26, causing an increase or decrease of the drive signal frequency and thus changing the current delivered to the lamp. Because this type of circuit provides dimming using a control signal that is separate from the line power, it is referred to as a “three-wire” type of dimming ballast.
An example of a “two-wire” type analog dimming ballast is provided in FIG. 2. This circuit is similar to that of FIG. 1, except that the control signal provided to the control circuit 28 is taken from the rail output of the rectifier 10. This type of circuit is typically used where power is supplied by a triac, which is conventionally used to provide dimming of incandescent lights by controlling the power supplied to the light. In the ballast of FIG. 2, the control circuit 28 controls the frequency of the drive signal in response to the power supplied by the triac.
Recent development efforts in fluorescent lighting have focused on reducing the size of ballasts so that, for example, a ballast and a fluorescent tube may be combined in a package that is compatible with conventional incandescent light sockets and fixtures. For such packaging, it is desirable that the size, weight and power consumption of ballasts are reduced as much as possible. Analog ballasts of the type shown in FIGS. 1 and 2 are relatively bulky and inefficient due to their large numbers of inductive windings and cores, and other circuits typically provided in ballasts such as preheat and ignition circuits add additional size and weight. Therefore a ballast that eliminates conventional analog ballast components would be highly desirable.